Camshaft phasers for varying the timing of combustion valves in internal combustion engines are well known. A first element, known generally as a sprocket element, is driven by a chain, belt, or gearing from the internal combustion engine's crankshaft. A second element, known generally as a camshaft plate, is mounted to the end of a camshaft of the internal combustion engine. A common type of camshaft phaser used by motor vehicle manufactures is known as a vane-type camshaft phaser. U.S. Pat. No. 7,421,989 shows a typical vane-type camshaft phaser which generally comprises a plurality of outwardly-extending vanes on a rotor interspersed with a plurality of inwardly-extending lobes on a stator, forming alternating advance and retard chambers between the vanes and lobes. Engine oil is supplied via a multiport oil control valve, in accordance with an engine control module, to either the advance or retard chambers, to change the angular position of the rotor relative to the stator, and consequently the angular position of the camshaft relative to the crankshaft, as required to meet current or anticipated engine operating conditions.
While vane-type camshaft phasers are effective and relatively inexpensive, they do suffer from drawbacks such as slow operation at low engine speeds due to low oil pressure, slow operation at low engine temperatures due to high oil viscosity, increased oil pump capacity requirement for the oil pump used to lubricate the internal combustion because the same pump is used to actuate the vane-type camshaft phaser, and the total amount of phase authority provided by vane-type camshaft phasers is limited by the amount of space between adjacent vanes and lobes and may not be sufficient to provide the desired amount of phase authority. For at least these reasons, the automotive industry is developing electrically driven camshaft phasers.
One type of electrically driven camshaft phaser being developed uses a harmonic drive gear unit, actuated by an electric motor, to change the angular position of the camshaft relative to the crankshaft. One example of such a camshaft phaser is shown in United States Patent Application Publication No. US 2012/0312258 A1 to Kimus et al. While the camshaft phaser of Kimus et al. does not use oil to actuate the camshaft phaser, oil is used for lubrication of various element of the camshaft phaser. Accordingly, oil is supplied under pressure to the camshaft phaser where the oil lubricates various elements within the camshaft phaser. After lubricating the various elements, the oil which drains out of the camshaft phaser through various interfaces is allowed to reach a drive member, such as a chain or belt, which transfers rotational motion from the crankshaft to the camshaft phaser. While this may be acceptable to some drive members, particularly chains and gears, other drive members, particularly belts, may not tolerate exposure to oil.
What is needed is an electrically driven camshaft phaser which minimizes or eliminates one of more of the shortcomings as set forth above.